1. Field of the Invention
The present invention relates to an apparatus for detecting physical marks from a wobble signal and a method thereof, more particularly to an apparatus capable of detecting physical marks by means of several threshold values or several offset values.
2. Description of the Related Art
When the mold of a recordable optical storage medium is engraved with grooves, the shallow grooves start from the center of the optical storage and are spirally formed thereon by means of a laser beam. Known as a pre-groove, the groove does not look like a smooth spiral; instead, it is a perturbed spiral that wobbles in a sinusoidal waveform of tiny amplitude. A signal read from the pre-groove is hereinafter referred to as a “wobble signal.” Every sector on an optical disc contains time-related data to provide address information for positioning the pick-up head.
Since the address code information is provided by the wobbling pre-groove, such information is called ADIP (Address In Pre-groove) data. To produce a recordable optical disc, the disc producing machine encodes the physical address information, and then the encoded data are converted into wobble signals, which are recorded on the tracks of the recordable optical disc, by minimum shift keying modulation (hereinafter referred to as “MSK modulation”), harmonic modulated wave modulation (hereinafter referred to as “HMW modulation”), etc.
Later on, to retrieve the recorded data, the optical storage device demodulates the wobble signals first and then decodes them in order to read the corresponding physical address information for accurately positioning the pick-up head.
FIG. 1 is a schematic diagram of the structure of an ADIP address word recorded on the pre-grooves of a Blu-ray disc after an MSK modulation. An ADIP address word 10 is represented by a data matrix of 56 columns and 83 rows, wherein each row is named an ADIP unit, and each element is equivalent to a wobble length. If three continuous wobble lengths respectively show data of 1,2 and 3, they represent a physical mark 11. The physical mark 11 is based on the MSK modulation, so it is also named an MSK mark. Physical address information consists of a plurality of MSK marks.
FIG. 2(a) is a functional block diagram of a conventional apparatus for detecting physical marks existing in a wobble signal. A wobble signal is converted into a matched signal by the matching value operation unit 21 of a physical-mark detection apparatus 20, and the matched signal is compared with a fixed threshold value by means of a comparator 22. The compared signal is decoded by a decoder 23 for obtaining wobble data with physical mark information.
FIG. 2(b) is a waveform diagram of signals converted from a wobble signal on a Blu-ray disc by a matching value operation unit 21. The signal at the first line is a normal clock signal with a plurality of monotone square pulses for the matching value operation unit 21 to refer to it for processing signals. On the other hand, the matching value operation unit 21 can also refer to a matching reference signal, at the third line, with a specific waveform. The product signal that the wobble signal is multiplied by the matching reference signal includes a physical mark 25′ within three continuous wobble length periods, as shown in the fourth line in FIG. 2(b). The physical mark 25′ corresponding with the physical mark 25 of the wobble signal represents an MSK mark. If the product signal is integrated or accumulated with a time interval to obtain a cumulating signal, a larger matching value 25″ can be obtained from the corresponding interval 25′ of the physical mark 25 in the wobble signal. On the contrary, a smaller matching values is obtained from the other intervals excluding the interval 25′ by the integral of the product signal. The matching value is compared with the threshold value to decide whether a physical mark exists.
In addition to the application of the Blu-ray disc, the wobble signal data, as shown in FIG. 1, is obtained, and the physical-mark detection apparatus 20 is also applied to an HD DVD (High Density Digital Versatile Disc) disc and DVD+RW disc, both of which have similar wobble signals.
FIG. 3 is a waveform diagram of the compared signals of a wobble signal on a DVD+RW disc processed by the matching value operation unit 21. The signal at the first line is a normal clock signal, including a plurality of monotone square pulses, for the matching value operation unit 21 to refer to it. The wobble signal has a physical mark 31 occurring within four continuous wobble length periods, and the physical mark 31 and the normal clock signal are opposite in phase. A sync unit product signal is produced by multiplying the wobble signal by the normal clock signal, and the bit zero product signal and bit one product signal are also produced by the same multiplying operation.
The waveform, shown in FIG. 4, is resulted from the similar signal conversion applied to an HD DVD disc. The wobble signal has two physical marks 41, each occurring within 6 continuous wobble length periods. The sync unit product signal, unity unit product signal, address 000 product signal and address 001 product signal are produced by multiplying the wobble signal by the normal clock.
The physical-mark detection apparatus 20 merely employs the fixed threshold value 24 to decide whether a physical mark exists or not, but occasionally the decision result of existence or non-existence for a physical mark is erroneous due to the distortion of the wobble signal. In other words, no matter how the magnitude of the fixed threshold value 24 is employed, the erroneous decision result of the existence or nonexistence still occurs.